AITA for not cleaning up a mess that somebody else left?

So, about a couple of months back I house sat for 2 close friends as they went on their Honeymoon, they had a cat and a dog so the main point is to look after to them and to quote "leave the house in a state that we left it for you". Honestly I didn't do much cleaning till the last day, where I did everything in one go (I did forget to do a few things here and there, but they weren't that bothered about that) now, when I first got to the house, they had already left, but they left a solid amount of uncleaned plates, bowls, forks, knifes, glasses, really a decent amount of stuff there. I was pretty annoyed that they left it all for me to clean up, the amount has been argued between me and them about how much there actually was, they say it was next to nothing, just a couple of plates and a glass but I know for a fact that wasn't the case, but I can't really argue that point because neither of us would agree on the amount, long story short, I left them there, if I'm being honest I was a bit spiteful about it, I'm willing to admit I'm in the wrong there (and I have apologized about that) however they basically didn't talk to me for a month and when I asked why they basically had a massive go at me for this. I do want to make it clear, if it was as much as they said it was I would have begrudgingly cleaned them, but I am wrong to think that them just leaving that for me to do is unacceptable? I made sure nothing was getting moldy or rotting because that would have caused more issues then it was worth, and if people on here say I should have cleaned them, I'll happily admit I'm wrong and use it as a learning experience, but is them leaving that mess for me okay to do? They said its a rule of house sitting, which when I ask close friends, they all agree them leaving there stuff was all wrong, but obviously they are gonna be bias, so here I am looking for an unbiased opinion.

TL;DR - I house sat for friends who left a bunch of pottery to clean, I refused to do it because I thought the amount they left was unacceptable and quite cheeky, they got upset since I left it to them.

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EDIT: Since it was asked pretty quickly, I wasn't paid to house sit, I was given IIRC £75 for food for the time I was there, which I did use all of, I didn't get paid for my time.